For persons with impaired hearing, the hearing aid devices which are mainly used today are those based on the principle that the sound is amplified and fed into the auditory meatus and stimulates the eardrum from the outside. In order to prevent feedback problems in these devices, the auditory meatus is almost completely plugged by a hearing plug or by the hearing aid device itself. This causes the user a feeling of pressure, discomfort, and sometimes even eczema. In some cases it even causes the user problems like running ears due to chronic ear inflammations or infections in the auditory canal.
For persons who cannot benefit from traditional, air conduction hearing aids due to such problems that have been described here it is previously known to use hearing aids which leave the auditory meatus free, see for instance U.S. Pat. No. 5,411,467 and U.S. Pat. No. 5,318,502 which hearing aids are both connected to the middle ear. Such a connection, however, requires a surgical operation in the middle ear which is a relatively complicated procedure.
By U.S. Pat. No. 5,282,858 and U.S. Pat. No. 4,988,333 it is also previously known to install a part of the hearing aid device on the middle ear bones. Although such a solution leaves the auditory meatus free, it nevertheless requires an extensive surgical installation procedure on the middle ear bones. These types of hearing aids have therefore not been used so much.
However, there are other types of sound transmitting hearing aids on the market, i e bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an implanted titanium screw installed in the bone behind the ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i e the hearing aid works whether there is a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461 and it is also referred to the BAHA® bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.
Even if the bone conduction hearing aid devices have made it possible for more people to benefit from a satisfactory hearing aid, there are also problems with this type of hearing aid devices. One problem is the permanent skin penetration which requires a good hygienic control and has aesthetic limits. By implanting parts of the apparatus hygienic as well as cosmetic aspects can be improved. Such a device is described in U.S. Pat. No. 4,904,233. A similar implantable bone anchored apparatus is also described in “Hearing by Bone Conduction”, Stefan Stenfelt, Chalmers University of Technology, 1999. It is also referred to our co-pending patent application PCT/SE01/01229 which relates to a hearing aid device which comprises an external part as well as an implantable part which is anchored on the outside of the skull bone so that it can be easily replaced without any advanced surgical operation.
A common feature for the hearing aid devices which have been described here is that vibratory generating means, vibrators, are required. Different types of vibrators are well known in the art. There are a number of known vibrator principles today in traditional as well as in bone anchored hearing aid devices it is normally used a vibrator principle which was described by Bell already in 1876. There is a detailed description of this principle applied on a bone anchored hearing aid device in “on Direct Bane Conduction Hearing Devices”, Technical Report No. 195, Department of Applied Electronics, Chalmers University of Technology, 1990.
It is also referred to Swedish Patent No. 85.02426-3 which describes a vibrator having means for damping the natural frequency of the vibrator.
In headphones for air conduction hearing aids any type of the so-called “Balanced Armature” principle is often used, see for instance U.S. Pat. No. 905,781, Baldwin 1908. Even the so-called Moving coil principle, known from conventional loud-speakers, could be used.
For vibrators used for bone conduction hearing aid devices there are specific requirements. The vibrators should be powerful enough for transmitting the vibrations to the skull bone and forward the vibrations through the skull bone to the inner ear without any surgical operation in the bone. If a part of the hearing aid device is implantable onto the skull bone the vibrator should be as small and compact as possible.
The existing vibrator types like Bell, Balanced armature, Floating mass and Moving coil principles can be used also in this type of implantable bone conduction hearing aid devices, but they do not always give an optimal function for this specific application.